As a background art of the present technical field, JP-2007-272292-A (Patent Document 1) is available. In this document, a shadow boundary extraction method is disclosed in which a pattern indicative of a relationship between pixel values of edge pixels belonging to an edge of an image pickup target in an image picked up by an image pickup apparatus and a plurality of pixels existing in the proximity of the edge pixels is calculated first. Then, a shadow boundary pattern in which a relationship between pixel values of shadow boundary pixels belonging to a shadow boundary between a shadow region and a non-shadow region in the image and a plurality of pixels existing in the proximity of the shadow boundary pixels is referred to, and the shadow boundary pattern and the calculated pattern are compared with each other. Then, the edge pixels belonging to the calculated pattern that coincides with the shadow boundary pattern are decided as pixels existing on the shadow boundary.